Gate valves serve very beneficial functions in many industries including the petroleum and chemicl industries. One attribute of gate valves is that the gate port may be made large so that a full opening through the valve is provided when the valve is opened. One problem frequently encountered in the case of gate valves is lack of formation of adequate seals between the gate or gates and the seat or seats, and gate valves are prone to leak unless adequate sealing means are provided. This invention seeks to provide an adequate seal for gate valves in the form of a positive cam actuated resilient secondary fluid seal.